This invention relates to active matrix electroluminescent display devices comprising an array of electroluminescent display pixels. In particular, the invention relates to an active matrix electroluminescent display device comprising an array of display pixels each comprising an electroluminescent display element and a driving device for controlling the current through the display element in a drive period based on a drive signal applied to the pixel during an address period preceding the drive period and stored as a charge on a storage capacitance associated with the driving device.
Matrix display devices employing electroluminescent, light-emitting, display elements are well known. The display elements may comprise organic thin film electroluminescent elements, for example using polymer materials, or else light emitting diodes (LEDs) using traditional III-V semiconductor compounds. Recent developments in organic electroluminescent materials, particularly polymer materials, have demonstrated their ability to be used practically for video display devices. These materials typically comprise one or more layers of an electroluminescent material, for example a semiconducting conjugated polymer, sandwiched between a pair of electrodes, one of which is transparent and the other of which is of a material suitable for injecting holes or electrons into the polymer layer. The polymer material can be fabricated using a CVD process, or simply by printing or a spin coating technique using a solution of a soluble conjugated polymer.
Organic electroluminescent materials exhibit diode-like I-V properties, so that they are capable of providing both a display function and a switching function, and can therefore be used in passive type displays.
However, the invention is concerned with active matrix display devices, with each pixel comprising a display element and a switching device for controlling the current through the display element. Examples of an active matrix electroluminescent (EL) display devices are described in EP-A-0653741 and EP-A-0717446. Unlike active matrix liquid crystal display devices in which the display elements are capacitive and therefore take virtually no current and allow a drive signal voltage to be stored on the capacitance for the whole frame period, the EL display elements need to continuously pass current to generate light. A driving device of a pixel, usually comprising a TFT, (thin film transistor), is responsible for controlling the current through the display element. The brightness of the display element is dependent on the current flowing through it. During an address period for a pixel, a drive (data) signal determining the required output from the display element is applied to the pixel and stored as a corresponding voltage on a storage capacitance which is coupled to, and controls the operation of, the current controlling drive device with the stored voltage serving to maintain operation of the switching device in supplying current through the display element during a subsequent drive period, corresponding to a frame period, until the pixel is addressed again.
A problem with known organic electroluminescent materials, particularly polymer materials, is that they exhibit poor stability and suffer ageing effects whereby for example the light output for a given drive current is reduced over a period of time of operation. While in certain applications such ageing effects may not be critical, the consequences in a pixellated display can be serious as any slight variations in light output from pixels can easily be perceived by a viewer.
It is an object of the present invention to provide an active matrix electroluminescent display device in which this problem is overcome at least to an extent.
In the absence of developments in the electroluminescent materials themselves to improve their stability, it is believed that electronic techniques can be employed to provide appropriate electrical correction for the effects of such degradation.
According to the present invention there is provided an active matrix electroluminescent display device comprising an array of display pixels each of which comprises an electroluminescent display element and a driving device for controlling the current through the display element in a drive period based on a drive signal applied to the pixel during an address period preceding the drive period and stored as a charge on a storage capacitance associated with the driving device, which is characterised in that each pixel includes electrooptic discharging means coupled to the storage capacitance for controlling the amount of light output from the pixel in the drive period which discharging means is responsive to light produced by the display element during the drive period and arranged to leak charge from the storage capacitance at a rate dependent on the display element light output.
Thus, a given stored signal voltage for determining a desired light output level of the display element of a pixel following addressing is progressively changed in the drive period according to the light output characteristic of the pixel""s display element through operation of the discharging means in the drive period, with the light output acting as a feedback variable, whereby the operation of the driving device controlling energisation of the element (and hence light output therefrom) in the driving period is correspondingly progressively adjusted. The proportion of the available drive period for which the display element is energised to produce light output is therefore dependent on, and regulated by, the action of the discharging means in discharging the storage capacitance according to its light output. In this way, the integrated light output from a display element in a frame period can be controlled so as to counteract the effects of ageing and improved uniformity of display output is obtained even though the degradation of individual display elements differs.
In order to obtain approximately a similar maximum amount of light from a degraded display element in the drive period, which corresponds approximately to a frame period less the row address period, the amount of charge initially stored in the storage capacitance in the address period may be increased slightly compared with that in the known display device by increasing the magnitude of the data signal appropriately so that a similar number of photons to that produced from an undegraded display element can be obtained from the degraded display element before the value stored on the storage capacitance is reduced through operation of the discharging means, at a rate dependent on the light emission of the display element, until the driving device begins to turn off. Alternatively, the drive voltage applied to the display elements can be adjusted appropriately. Thus, the amount of light produced by the degraded (aged) display element can be maintained similar to that from the display element before degradation.
The driving devices of the pixels preferably comprise TFTs and may be either n type or p type TFTs, for example polysilicon MOS TFTs. References herein to discharging should therefore be construed appropriately in relation to the nature of the charge stored on the storage capacitances in the address phase for both cases.
The discharging means preferably comprises a photoresponsive element in the form of a photodiode which photodiode is connected to the storage capacitance and arranged to be reverse biased in the drive period so as to leak charge from the storage capacitance in response to light from the display element falling thereon. Although it is envisaged that a photoresponsive device other than a photodiode and operable in response to light falling thereon to leak charge from the storage capacitance at a rate dependent on the level of the incident light in the drive period may alternatively be used, a photodiode is preferred for this purpose as its operation in leaking is independent of the voltage across it and substantially linearly proportional to incident light level.
With the driving device comprising a current-controlling transistor (TFT) connected in series with the display element between two supply lines at different voltage levels, and with the storage capacitance being connected between the gate node of the transistor and one of the supply lines, as in the known device, then the photodiode can be connected, with appropriate polarity to be reverse biased, in parallel with the storage capacitance between the gate node and that supply line. Photocurrents generated in the photodiode by light from the display element result in leakage through the photodiode of the stored charge and gradual reduction of the voltage at the gate node.
In a comparatively simple embodiment of the invention, the discharging means may solely comprise a photodiode operating in the aforementioned manner. Such an arrangement would be beneficial in overcoming to a useful extent problems due to display element degradation in many situations.
However, if merely a photoresponsive element such as a reverse-biased photodiode connected across the storage capacitance is used as the discharging means, problems may be experienced. As a result of the operation of the discharging means in discharging the storage capacitance, the level of light produced from the display element gradually diminishes during the drive period and this will lead to the display element being turned off in a slow manner which may not be adequately precise for the regulation control desired. At comparatively low light output levels the operation of the pixel would become less-well controlled. Typically the characteristics of a photodiode are such that it becomes much less efficient at relatively low light levels. Also, in the case where the driving devices comprise TFTs, the operation of an individual driving device becomes less well defined as the gate voltage begins to approach its threshold voltage and so non-uniform operation of the devices of the array could occur.
In a preferred embodiment of the invention, therefore, the discharging means is further arranged to rapidly discharge the storage capacitance and curtail light output from the display element at a point in the drive period which is controlled by, and dependent on, the operation of the display element. The operation of the discharging means in this respect is preferably determined by an operational characteristic indicative of the light output of the display element dropping to a certain, lower, level. Consequently, problems due to the nature of the behaviour of a photo-responsive element such as a photodiode at low light levels and the operation of the driving TFT close to its threshold voltage are avoided. The discharging means could be made responsive directly to the light output of the display element for this purpose but, preferably, the operation of the discharging means in this respect is made dependent on an electrical parameter that varies in accordance with the drive level of the display element, for example according to the level of electrical current flowing through the display element or a voltage in the pixel circuit which varies in accordance with such current (the light level of the display element being dependent on this current). The controlled point is then determined by the electrical parameter reaching a predetermined level. This enables electrically, rather than optically, responsive switching devices or circuits to be used for this purpose.
Because the light output from the display element is suddenly terminated rather than diminishing very gradually, grey scales are made easier to control and efficiency is improved.
In this embodiment, the discharging means may comprise a photoresponsive element, again preferably a reverse-biased photodiode, connected across the storage capacitance and responsive to light generated by the display element to leak away charge stored on the storage capacitance slowly, and a switching device, such as a transistor, which is connected in parallel with the photoresponsive element across the storage capacitance and responsive to current flow through the display element to discharge the storage capacitance rapidly upon the level of current flow through the display element reaching a certain, low, level.
Preferably, however, the discharging means comprises a photoresponsive transistor connected across the storage capacitance through which charge is leaked away by photocurrents generated therein by light from the display element and whose gate is coupled to a source of potential dependent on the current flowing through the display element. One current carrying electrode of the transistor, for example the drain junction, can be arranged reverse biased and responsive to light falling thereon so that the drainxe2x80x94source path behaves as a reverse-biased photodiode while the transistor is off and with the potential applied to its gate controlling its switching operation. Thus, only one device is needed to fulfil the required functions for the discharging means of, initially, slow and, subsequently rapid discharge of the capacitance. Such a transistor is relatively simple and convenient to fabricate alongside, and simultaneously with, the transistors of the active matrix circuit, e.g. the driving TFTs.
Conveniently, the gate of the photo-responsive transistor may be coupled to the node between the display element and the driving device. The voltage at this node varies according to the level of current flow through the display element. As the storage capacitance discharges due to photocurrent in the photo-responsive transistor, the gate voltage decreases and the voltage across the display element increases until at a certain point the photoresponsive transistor""s threshold level is reached causing it to turn on and rapidly discharge the capacitance.
When using a TFT as the driving device of a pixel, the invention offers a further important advantage. Since the drive current for a display element is determined by the voltage applied to the gate of the TFT, corresponding to the voltage stored in the capacitance, this drive current depends strongly on the characteristics of the TFT and so any variations in the threshold voltage, mobility and dimensions of the individual TFTs of pixels over the array, for example due to manufacturing process tolerances, can produce unwanted variations in the display element currents and hence output light levels produced, which causes non-uniformities in the display output. The effect of the discharging means in controlling the stored voltage signal will also compensate to an extent for such variations in TFT characteristics.
A further advantage is that the operation of the discharging means can also negate at least to some extent problems due to voltage drops occurring during the drive period in a common current line connected to, and shared by, the display elements of, for example, all the pixels of a row. Although the invention is particularly beneficial in devices using polymer LED materials, it can of course be applied to advantage in any active matrix electroluminescent device in which the electroluminescent material used similarly suffers ageing effects resulting in lower light output levels for a given drive current over a period of time of operation.